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History of Atomic Theory
Study Guide
Created by Natalie Cielanga
Discover the ideas that led to our current understanding of the struc-
ture of the atom. This journey begins well over 2000 years ago in Asia
with categories of matter and continues through the Dark Ages where
the quest for riches and everlasting life was sought through alchemy.
These early concepts and accidental discoveries are the beginning of
modern science and our current understanding of the minute particles
that are the foundation of all matter.
Early Concepts About Matter
Five Element Theory
Some of the earliest recorded ideas about
matter come from China and India. Philos-
ophers developed five categories of matter:
fire, water, earth, metal and wood. This
system did not include the idea of atoms
but we can see an attempt to differentiate
matter into categories. In chemistry we
classify matter based on phases (gas, liquid
and solid) or based on the atomic make-up
(mixture, pure substance or element).
The Five element theory is still alive and
many alternative therapies connect to it.
Democritus gives atoms the name ‘Atomos’
Democritus lived in Greece in 4th century BC. He developed a theory of matter
based on ‘atomos’ which he described as tiny indivisible particles. His theory was
that the atoms that made different materials were physically different from one an-
other such as in the images below. Other philosophers of that time that that sub-
scribed to the idea of atoms include Leucippus and Lucretius. Lucretius wrote a
poem which has been translated into English called ‘On the Nature of Things’
which includes the idea of atoms. Even though many philosophers subscribed to
the idea of atoms, these views were not widely accepted until the 16th century.
Why do you think the idea of atoms took so long to become the dominant theory? You may want to use
other sources such as the link provided below to gain more information to answer this question.
https://courses.lumenlearning.com/cheminter/chapter/democritus-idea-of-the-atom/#:~:text=Democritus.&text=Aristotle%
The Email Lab
It is difficult for us to imagine how challenging it would have been to come up with the
idea of the atom since this concept is ubiquitous in our education, but we can observe
other ways of trying to connect pieces of information or develop a story with limited infor-
mation. Read four emails from the email lab in the appendix and create a story or two of
how they fit together. Consider the details of the emails such as dates, locations, names
etc. Next choose two more emails from the appendix and consider if they fit with any of
the stories that you have proposed. Keep adding emails and adjusting your story.
2000 Years of Alchemy: Failed Attempts to Create Gold
Alchemy can be described as a pseudoscience or medieval chemistry that was pre-
dominantly interested in turning easily available metals into gold and finding univer-
sal cures. Through trial and error, alchemists discovered elements such as mercury,
sulphur and antimony.
An alchemist named Hennig Brand of the 17th century, accidently discovered phos-
phorus while trying to create gold. He thought the golden colour of urine indicated
that urine contained gold so he collected large amounts of urine and boiled it down.
(Imagine how unpleasant this would have smelled!) Hennig failed to find gold but he
discovered something that was new to world of alchemy and he named it
‘phosphorus’. He kept it secret for six years before sharing his discovery.
There were many alchemists in this time period that were doing their own experi-
ments to find gold and/or cures to people’s ailments. The number of failed experi-
ments may have been huge but most of these weren’t recorded. There may have been
many other successes similar to Hennig’s but alchemists were very secretive because
they did not want to share their knowledge with their competitors. Hennig’s failure to
find gold has been recorded because he eventually shared his discovery and his dis-
covery of phosphorus is important and relevant to modern society.
From Alchemy to Chemistry
Contributions from alchemy include metalworking, ceramics, dyes, and extracts. Alt-
hough it brought many valuable discoveries the practice was losing favor in the 17th
century to chemistry. Chemistry is detached from spiritual beliefs and uses a precise
and empirical framework based on the scientific method.
Robert Boyle (1627—1691) is often stated as being the father of chemistry. He was born
into a very wealthy British Anglican family, had a studious nature and was well-
educated. Since he was well-educated he had access to philosophical writing from an-
cient times and more recent. He believed in a form of atomism called corpuscularism.
Corpuscularism claims that everything is made of minute particles of a single universal
matter.
Boyle made many important contributions to theology and philosophy. Some of his
most important contributions to chemistry include:
experiment on the properties of air, discovering that air has mass and is a mixture
Wrote ‘The Sceptical Chymist’, a writing that promoted the idea that matter was
made of minute particles and presented methods of chemical analysis.
Discovered the relationship between air pressure and volume, known as ‘Boyle’s
Law’
Research Robert Boyle and answer the following questions:
1. Who were the scientists and philosophers that he conversed with, learned from
and shared his ideas with?
2. What other discoveries were being at this time related to chemistry that would
have influenced Boyle?
A Tale of Two Scientists
No two scientists are alike. They differ in their personalities, influences and approaches
to scientific experimentation. An example of two scientists that are starkly different and
yet both made important contributions to the advancement of scientific knowledge are
Antoine Lavoisier and Joseph Priestly.
Joseph Priestly (1733—1804) lived in England until 1794 where he worked as a minis-
ter. He was keenly interested in science and experimented with electricity and gases.
He believed that the accumulation of facts was more important for scientific progress
than the insights of a few great thinkers. He has been described as being a maverick for
his independent and unorthodox views and approaches to experimentation. His contri-
butions to chemistry include the relationship between electricity and chemical change
and the discovery of oxygen.
Antoine Lavoisier (1743—1794) lived in France and worked as a tax collector. He was
very interested in scientific experimentation and his wealth provided him the ability to
design and build expensive apparatus and pay talented researchers to work in his lab.
His experiments were largely focussed on quantitative analysis. Through quantitative
analysis he established the law of conservation of mass and determined that combustion
and respiration are caused by chemical reactions.
The two men met in 1774 and Priestly shared information on his experiments with air.
Lavoisier repeated Priestly’s experiment and through the process discovered and named
the element oxygen.
The documentary ‘The Mystery of Matter: Out of Thin Air’ provides a charming portrayal
of these two scientists. The documentary is available on youtube through the following
link:
https://www.youtube.com/watch?v=z3Gt5IOjAuc
Antoine Lavoisier is often pictured
with his young wife, Marie Anne
Paulze. Marie was known to be
highly intelligent and assisted An-
toine in his scientific experiments.
The Modern Atomic Theory
In 1808, John Dalton published the first complete attempt to describe all matter
through the lens of atoms. It was called ‘A New System of Chemical Philosophy’ and in-
cluded the following four statements:
1. Each element is made up of tiny particles called atoms.
2. The atoms of a given element are identical; the atoms of different elements are dif-
ferent in some fundamental ways.
3. Chemical compounds are formed when atoms combine with each other. A given
compound always has the same relative numbers and types of atoms.
4. Chemical reactions involve reorganization of the atoms—changes in the way they
are bound together. The atoms themselves are not changed in a chemical reaction.
The theory about matter that Dalton put forth is mostly consistent with how we under-
stand matter today. The theory builds on the principles laid out by other chemical sci-
entists of the time such as Cavendish, Proust and Lavoisier as well as physical scientists
such as Newton. What is remarkable about Dalton’s Theory is that it goes beyond what
could be deduced from experimental evidence of the time and it holds true today. Dal-
ton had abolished the idea that was held for thou-
sands of years that atoms of all kinds of matter are
alike and proposed that atoms of different elements
have different sizes and mass.
Acceptance of the atomic theory was slow but he was
recognized in his lifetime. He is considered to be one
of science’s great thinkers and differs from the style
and approaches of Joseph Priestly and Antoine La-
voisier.
This list of elements was created by John Dalton and
was included in 1808.
He attempted to determine the masses and some
combinations of elements that formed compounds
What parts of Dalton’s theory do not agree with
our current understanding of atoms and com-
pounds?
Roles, Expectations and Constraints
Throughout history we can find examples of people that moved outside the roles and ex-
pectations that society had for them. A man born into a wealthy family in the 17th cen-
tury, for example, had different opportunities and limits than a man born into a working
class family. For women, the roles and constraints were more limiting than for men.
Women had less access to education, fewer options for living independently and less po-
litical power than men. Despite these constraints, some women were able to move out-
side of their expected roles and make great contributions to science.
Choose one or more of the following female scientists to research and answer the question
‘What factors and influences were important for her to overcome the constraints on wom-
en of her time and place?’
Lise Meitner (1878—1968) was a physi-
cist who made important discoveries in
nuclear fission.
Marie Curie (1867—1934) was a chemist
who conducted pioneering research on
radioactivity.
Ada Lovelace (1815—1852) has been
called ‘the first computer programmer’
and invented the first algorithm.
Mary Sommerville (1780—1872) was a science
writer and did original research on sunlight
and its magnetizing effects
Confusion Reigns Over Atomic Masses and Compounds In the early 19th century French chemist Joseph Gay-Lussac (1778—1850) was perform- ing experiments that utilized the relationship between temperature, pressure and vol- umes of gases. For example, he found that 2 volumes of hydrogen react with 1 volume of oxygen to form 1 volume of gaseous water. He also recorded other examples of reactions of gases. An Italian chemist named Amedeo Avogadro (1776—1856) interpreted Gay- Lussac’s results and made the following hypothesis: ‘At the same temperature and pressure, equal volumes of gases contain the same number of particles’ Based on this hypothesis, the following relationship would be true: This representation shows the volume of the water is the same as the volume of the hy- drogen gas and the number of particles of hydrogen are the same as the number of parti- cles of gaseous water. This hypothesis was not accepted by most chemists at the time because of a belief that only atoms of different elements could attract one another and therefore, diatomic molecules such as the hydrogen and oxygen molecules could not ex- ist. Avogadro died before his hypothesis was accepted Many scientists were measuring masses of elements and compounds during the first half of the 19th century. However, there was no consensus about the interpretations of these measurements and many different tables of atomic masses were proposed. This confu- sion was cleared up by Italian chemist, Stanislao Cannizzaro (1826—1910). Cannizzaro believed that Avogadro’s hypothesis was correct. He proceeded to collect data on relative masses of elements and compounds. He collected such large quantities of data and re- ported consistent results that eventually the scientific community came to agree that his interpretations made sense. His work led to the approximate values of the atomic mass- es. Today, Avogadro is recognized with the number 6.02 x 1023 It is called Avogadro’s num- ber and it represents the number of atoms or molecules in a mole of substance.
The Gas Laws
Many of the early experiments that allowed scientists to better understand atoms and
chemical reactions were done with gases. The PhET simulation linked here, called ‘Gases
Intro’ is great tool to experiment with to learn the relationships between gas volume, pres-
sure and temperature.
https://phet.colorado.edu/en/simulation/gases-intro
Subatomic Particle is Discovered
Electricity had been discovered by Benjamin Franklin in 1750 and he proposed that an
electrical current consisted of a stream of extremely small particles but it wasn’t until the
late 19th century that the small particles were named electrons. J.J. Thomson (1856–
1940) was experimenting with a cathode ray tube. The technology behind the cathode
ray tube had been around for over 100 years, it consisted of an electrical machine and an
air pump. Many inventions came from the combination of these two technologies includ-
ing flourescent light bulbs and TV picture tubes but Thomson combined them to discover
the electron. He ran an electrical current through a vacuum chamber and deflected the
current with a small magnet. Go to the link on the Physics Aviary Website provided here
to play with a simulation of the cathode ray tube.
https://www.thephysicsaviary.com/Physics/Programs/Labs/ThompsonetomLab/
index.html
Thomson’s genius was in drawing profound conclusions from simple observations. From
the cathode ray tube experiment he reasoned that electrons were produced from the met-
al electrodes therefore all atoms must contain electrons. Also, since atoms were known
to be electrically neutral, he thought they must also contain a positive charge. The plum
pudding model of the atom was born.
Plum pudding was a popular
English dessert that included
raisins in a pudding mixture.
Thomson’s model of the atom
had electrons, like raisins, in
a positively charged cloud,
like the pudding. Sometimes
this is referred to a blueberry
muffin model since this is
more relatable today than
plum pudding.
Changing the Concept of the Atom’s Structure Ernest Rutherford (1871—1937) was a junior assistant in Thomson’s lab when Thom- son discovered the electron. Despite being directly influenced by Thomson and the exciting work on discovering subatomic particles, Rutherford’s research interest was stronger in radioactivity. Radioactivity had been recently discovered in France and was being researched by Marie Curie as well as many other scientists. Three types of radioactive emissions had been identified at this time: gamma rays, beta particles and alpha particles. The alpha particle proved to be crucial in Rutherford’s research. Al- pha particles consist of two protons and two neutrons bound together creating a posi- tively charged particle. However, in the early 20th century all that was known about them is that they were positively charged and had a mass about 7300 times the mass of an electron. For a more in depth look at alpha particles, try this simulation from PhET: https://phet.colorado.edu/en/simulation/legacy/alpha-decay Rutherford was studying the scatter patterns of alpha partilces from uranium sources. He found that most alpha particles travel straight with some minor deflection after passing through thin targets. When he moved the detector next to the source of the beam of particles, he was astonished to see the some particles reflected back. He not- ed that it was as surprising as shooting a cannonball at a piece of paper and seeing the cannonball bounce back. These results did not fit with Thomson’s plum pudding model of the atom, which would have resulted in the alpha particles passing straight through the gold foil. Rutherford spent nearly a year contemplating the results before concluding that the atom must have a highly dense, positively charged nucleus. The simulation called ‘Rutherford Scattering’ on the PhET website is useful to see how Rutherford’s experiment worked: https://phet.colorado.edu/en/simulation/rutherford-scattering The above figure shows Rutherfords gold foil experiment (A) and the difference between between Thomson’s model of the atom and Rutherfords (B)
The Challenge of Conceptual Change There are multitudes of examples in science history of a concept taking hold only to be replaced by a new concept. This process often takes a lot of time. Avogadro’s new idea about gases did not get accepted by the broader scientific community because it conflict- ed with a mistaken concept that atoms that are alike cannot attract each other to make molecules. Unfortunately Avogadro did not live long enough to see his hypothesis get ac- cepted. Lavoisier was challenged by the scientific community for his ideas about air since there was a pre-existing concept of phlogiston. He noted that the younger chemists fully accepted his theory but older scientists resisted it. When Rutherford saw the results from his gold foil experiment he took a year to propose the new concept of an atom as having a positively charged nucleus. During this time he would have repeated the experiment many times and examined it for errors. Was the equipment faulty? Was there another source of alpha particles in the room? Was the gold foil contaminated? After he exhausted the potential sources of error, he had to change his concept of the atom to fit with the experimental results. Changing a concept in our mind can be compared to renovating a house. A kitchen reno- vation may effect many other areas of the house if wiring and plumbing need to be updat- ed and design elements need to be carried into other parts of the home. A concept in our mind is connected to other concepts so changing one will create change elsewhere. If the concept change is really big it can be compared to creating a new travel corridor in a city. The process takes a lot of effort and time and has impacts that are difficult to foresee. As learners, we are constantly faced with conceptual change. Sometimes the instruction- al material is purposely designed with simple concepts being replaced by more complex ones. In chemistry we are first taught that chemical reactions are reactants changing to products and that all the molecules react. Next we are told to discard the idea that all molecules react and to determine the percent yield. Further along we are taught that many reactions go in both directions at the same time. Each time we have to deconstruct a previously held con- cept to make way
Quantized Energy By the end of the 19th century scientists were feeling confident about their understanding of matter and energy. Matter was thought to consist of particles and energy was de- scribed as a wave. Max Planck (1858—1947) was studying electromagnetic radiation profiles of solid iron that was heated to incandescence. The radiation profiles that were expected would have no maximum but that is not what was found. From this result, Planck hypothesized that energy could only be gained or lost in packets called ‘quantum’. The graph below is an example of blackbody radiation at different temperatures. As the temperature is increased, the shorter wavelengths increase. This helps to describe the vis- Shortly after Planck’s discovery, Albert Einstein (1879—1955) was observing how light shining on certain metals releases electrons. If light is made of waves, it didn’t make sense that a wave could knock an electron out of a metal atom but if he thought about light as particles in the form of bundles, then with enough energy it made sense that elec- trons were released. Einstein built on Planck’s concept. Planck was concerned with the emission of light in bundles and Einstein went further to propose that matter always con- sists of bundles.
Putting the Pieces of the Puzzle Together In the 19th century, scientists discovered that each element has its own unique emis- sion spectrum. Spectral analysis became an important tool to discover and identify new elements but why these patterns existed was not known. It is now understood that these patterns result from electrons transitioning from higher to lower energy states and releasing photons that correspond to the difference in energy between the higher and lower states. Each element has many possible transitions. Previous to quantum theory, it was thought that any atom could absorb any amount of energy and release any amount of energy. Atomic emission and absorption spectra demonstrate that energy is absorbed and released in discrete amounts. The energy lev- els of the electrons were not revealed through the atomic spectra, just the difference be- tween energy levels. The unique colours produced by chemical salts when exposed to a flame, are due to the emission spectra of the elements.
The First Model of the Atom to Incorporate Quantum Theory Niels Bohr (1885—1962) was the first scientist to apply the concept of quantum theory to the structure of the atom. The Bohr model of the atom has the following characteristics: principle energy levels (shells) hold the electrons electrons closer to the nucleus have lower energy electrons farther from the nucleus have higher energy each energy level has a maximum number of electrons it can hold a ground state atom has its energy in the lowest state possible Previous to this quantum model, a planetary model of the atom was hypothesized. The planetary model is easy to grasp since it is based on our solar system therefore this mod- el is still widely displayed in images of the atom today. Bohr’s model kept the circular or- bits of the planetary model but he related the radii to the discrete wavelengths in the emission spectra of hydrogen.
The Electron, the Photon and the Platypus At the end of the 18th century, Europeans were introduced to an exotic animal from Aus- tralia called the platypus. The animal’s unique attributes made it difficult for anatomists to categorize. It was thought to be a mammal because it was discovered to have mam- mary glands but then it was observed to lay eggs like birds, reptiles and fish. Until this time, animals fit nicely into the categories that anatomists created but these are human created categories to describe nature and nature does not follow human concepts. A new category was needed for this miraculous creature. The story of the platypus is the same in many ways to the story of the electron and the photon. For hundreds of years scientists have argued over which category light belongs to: wave or particle. Einstein finally proposed that it is both, breaking away from the cat- egories that were entrenched in people’s minds. Not long after, Schrodinger proposed the same fate for the electron. Photons and electrons are much more abstract than the platypus which makes them incompre- hensible in many ways. Heisenberg noted that the structure of the atom is inaccessible to observa- tion and that it can’t be described in familiar terms. When he was asked “How can we picture the atom?”, he responded “Don’t try.” The following website includes a couple of short videos that provide some descriptions for compre- hending the particle wave duality. https://www.britannica.com/science/wave- particle-duality
Waves in the Atom The challenge with the Bohr model of the atom was that it only worked for hydrogen. Louie De Broglie (1892—1987) improved upon the Bohr model by adding the wave nature of electrons. De Broglie created an equation that demonstrated the relationship between one of the wave-like properties of matter and one of its properties as a particle. He found that most particles are too heavy to observe their wave properties but an electron has a small enough mass to exhibit both particle and wave properties. When he applied his theory to the Bohr model of the atom he found that only certain specific orbits allowed the electron to satisfy both its particle and wave duality. The next model of the atom was developed by Erwin Schrodinger (1887—1961). He treat- ed the electron like a wave and describes regions in space, called orbitals, where electrons are likely to be found. This model doesn’t tell exactly where the electron is but where it is likely to be found. The following image demonstrates the De Broglie model of the atom. If you have been introduced to electron configuration of atoms and ions you should recog- nize the labels of the orbitals.
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